One of the current therapies for control of the elevated intraocular pressure, which seems to be related to the occurrence and progression of glaucoma, consists of the topical administration of carbonic anhydrase inhibitor. Spanish patent ES 2 053 738 describes, among others, some 4-(N-alkylamino)-5,6-dihydro-4H-thien-(2,3-b)-thiopyran-2-sulfonamide-7,7-dioxides of general formula (I):
wherein R1 is H or alkyl and R2 is alkyl, which are active as inhibitors of carbonic anhydrase when administered topically. Of particular relevance is the compound (4S-trans)-4-(N-ethylamino)-5,6-dihydro-6-methyl-4H-thien-(2,3-b)-thiopyran-2-sulfonamide-7,7-dioxide, denominated dorzolamide, of formula Ia:
Said Spanish patent describes several processes for obtaining compounds of formula (I), which include the following:
1) oxidation of the corresponding 4-(N-alkylamino)-5,6-dihydro-4H-thien-(2,3-b)-thiopyran-2-sulfonamide with aqueous oxone in an organic solvent:

2) reduction of the corresponding derivative containing an N-acyl group:

3) reacting the corresponding compound containing an hydroxy group in position 4 with toluenesulfonyl chloride followed by the addition of the desired alkylamine:

4) treating the corresponding compound containing a carbonyl group at position 4 with an amine in the presence of titanium tetrachloride, followed by reduction of the resulting intermediate with a metal hydride complex:

The aforementioned Spanish patent ES 2 053 738 describes the aforementioned processes for obtaining the cis or trans diasteroisomers, the levo or dextro enantiomers of said diastereomers or isomeric mixtures thereof.
The previously described processes have some drawbacks, which include:                when compound (I) is an enantiomer, for example, dorzolamide, the separation of the enantiomers is performed on the product of the last stage of synthesis, with the subsequent loss in the yield of the process because at least half of the material is lost when the entire synthesis has been performed;        in process 1) the last reaction is oxidation of the thioether group to a sulfone group, using oxone as an oxidant reagent; in these conditions, there is the risk of oxidation of the amine nitrogen, giving rise to by-groups that have to be removed during the purification of the final product;        process 2) comprises reduction of the amide group to amine as the last stage of the process, describing in said Spanish patent the use of diborane as a reducing agent, which has the drawback of having to perform the reaction under very energetic conditions in order to hydrolyse the borates which remain covalently bound to the product of the reaction; said reaction is performed by heating the reaction mixture under reflux with a mineral acid at a high concentration (for example, hydrochloric acid), which may give rise to the formation of by-products or alterations in the stereochemistry of the material with the subsequent potential problem of quality of the final product;        in process 3), the last reaction is a nucleophilic substitution reaction at the carbon in position 4; and, in the event that a specific geometric stereochemistry were required, as in the case of dorzolamide (trans), this reaction requires a complete inversion of the configuration because if not, it is necessary to separate the cis/trans mixtures formed, in addition to the separation of the enantiomers in the aforementioned final stage, with the subsequent loss of yield of the process; and        in process 4), separation of the cis/trans diastereomers formed during reduction of the imino group is required.        
Some of the problems mentioned above are solved in Spanish patent ES 2 112 482, where an enantioselective synthesis is described of compounds of formula (I), especially of dorzolamide, which uses the compound (4S-trans)-4-hydroxy-5,6-dihydro-6-alkyl-4H-thien-(2,3-b)-thiopyran as starting material and follows the synthesis scheme shown below:

Chiral hydroxysulfone, the starting material of the synthesis, can be obtained by processes described in European Patents EP 658 211 and EP 590 549 or in U.S. Pat. Nos. 5,391,772, 5,474,919 and 5,760,249. In these processes, the chiral hydroxysulfone is obtained by asymmetric enzymatic reduction of the corresponding ketosulfone or by cyclation of the chiral thienyl thiobutyric acid, obtained in turn from a chiral hydroxyester or lactone, and subsequent stereospecific reduction of the ketone formed.
The key stage in this process is the conversion of the hydroxysulfone into the acetamidosulfone. This reaction is carried out by means of a Ritter Reaction which, in this case, takes place with retention of the configuration. The subsequent introduction of the sulfonamide group and the following reduction of the amide group to an amine lead to the desired product. Despite the fact that this process resolves some of the problems posed earlier regarding the processes described in patent ES 2 053 738, it has the drawbacks related with the fact that the last stage of the synthesis is reduction of the amide group to an amine with diborane. For this reason, the final purification process is complex and laborious.